1. Field of Technology
The present invention relates to a sensor-less motor drive apparatus and motor drive method which do not require a non-activation period for rotor position detection.
2. Description of Related Art
Brushless motors are now generally used for the spindle motor in hard disk drive and optical disc drive devices, as well as for air conditioner fan motors and compressor drive motors. An inverter and PWM drive control are used to drive brushless motors in order to achieve variable speed control over a wide range of speeds and to reduce power consumption. Hall elements or other position sensors are normally disposed every 120 electrical degrees in a brushless motor with a three-phase winding in order to detect the magnetic pole position of the rotor. This brushless motor is substantially sinusoidal wave driven with the width of 180-degree activation using signals from these position sensors denoting the rotational position of the rotor.
Sensor-less drive technology has also been developed in order to reduce cost and size. One method of achieving sensor-less drive uses an energizing period of 120° or other angle less than 180°, and detects the zero cross of the induced voltage generated in the non-activation period. The presence of a non-activation period in this prior art method however, causes vibration at the activation changing timing and this vibration results in acoustic noise. Japanese Patent 3424307 addresses this problem and teaches motor drive control technology for detecting the rotor position without requiring a non-activation period for rotor position detection.
The content taught in Japanese Patent 3424307 is described briefly below with reference to FIG. 39 and FIG. 40. FIG. 39 is a block diagram showing the general configuration of a conventional motor drive apparatus. As shown in FIG. 39, this conventional motor drive apparatus has a power source 1p, the motor 10p that is driven, a drive unit 20p, a position detection unit 100p, and a microprocessor 200p. 
The drive unit 20p has a three-phase bridge circuit with power transistors 21p, 22p, 23p, 24p, 25p, 26p disposed between the power source 1p and ground GNDp, and switches according to an activation controlling signal from the microprocessor 200p to control driving the motor 10p. The position detection unit 100p detects the third harmonic component contained in the induced voltage to detect the rotor position. This detection operation is described in further detail below. The detected position detection signal PSp is input to the microprocessor 200p. 
The microprocessor 200p generates an activation controlling signal used to control driving the power transistors 21p to 26p of the drive unit 20p. More specifically, the microprocessor 200p generates a voltage pattern signal timed to the edge of the position detection signal PSp, and generates a control voltage according to the difference between a frequency acquired from the position detection signal PSp and the frequency of the frequency control signal Frp. The activation controlling signal is generated according to this voltage pattern signal and control voltage.
The foregoing motor drive apparatus can thus detect the rotor position without requiring a non-activation period for rotor position detection. This motor 10p enables sensor-less drive with the width of 180-degree activation.
The detection operation of the position detection unit loop is described in detail next with reference to the timing chart shown in FIG. 40.
The position detection unit 100p is composed of pseudo neutral point generator 90p having three resistors 91p, 92p, 93p, a differential amplifier 101p, an integrator 102p, a lowpass filter 103p, and a comparator 104p. One end of each of the resistors 91p, 92p, 93p is connected to a common node, and the other end is respectively connected to the node between a corresponding three-phase winding Lup, Lvp, Lwp and the drive unit 20p. The voltage produced at the common node is thus the pseudo neutral point voltage Vnp averaging the terminal voltages Vup, Vvp, Vwp of the motor 10p. 
The differential amplifier 101p differentially amplifies the neutral point voltage Vcp of the motor 10p and the pseudo neutral point voltage Vnp, and outputs difference signal Vdp. FIG. 40 shows the induced voltage Eup, Evp, Ewp occurring in phases U, V, W, and difference signal Vdp. The third harmonic component of the induced voltage can thus be detected as a result of the differential amplifier 101p differentially amplifying neutral point voltage Vcp and pseudo neutral point voltage Vnp, and the frequency of difference signal Vdp is three times the induced voltage frequency.
Difference signal Vdp is input to integrator 102p and integrated. The integrated signal Vdip output by the integrator 102p is shown in FIG. 40. As indicated by the offset component shown in FIG. 40, the cumulative offset is superimposed on the result of the integration operation. The integrated signal Vdip is then input to lowpass filter 103p and the DC component Vdilp is detected as shown in FIG. 40. The integrated signal Vdip output from the integrator 102p and the DC component Vdilp from the lowpass filter 103p are input to the comparator 104p, which compares the effect of the DC component. More specifically, the normal zero cross of the integrated signal Vdip can thus be detected. This zero cross is denoted by the solid dots on the integrated signal Vdip in FIG. 40. The comparator 104p then outputs the result of this comparison to the microprocessor 200p as the position detection signal PSp shown in FIG. 40. The position detection unit 100p can thus detect the normal zero cross point by comparing the offset component superimposed during integration of the difference signal Vdp with the DC component acquired by the lowpass filter 103p. 
The motor drive apparatus according to the prior art described above thus detects the third harmonic component of the induced voltage by differentially amplifying the neutral point voltage Vcp of the motor and the pseudo neutral point voltage Vnp, and detecting the rotor position using an integrator 102p, lowpass filter 103p, and comparator 104p. The microprocessor 200p also generates an activation controlling signal according to the detected rotor position, and provides sensor-less drive control of the motor 10p through drive unit 20p. 
The arrangement of the foregoing prior art leaves the following problems.
First, the method taught in Japanese Patent 3424307 enables position detection reflecting the offset component of the integrator 102p, but the effect of this is little when the comparator 104p has an offset. Because the third harmonic is typically small, the effect of the offset component of the comparator 104p cannot be ignored as the position detection signal will contain significant position detection error.
Furthermore, if the frequency of the integrated signal Vdip is lower than the cutoff frequency of the lowpass filter when the DC component of the integrated signal Vdip is detected by the lowpass filter, it will not be possible to correctly detect the DC component. In this case, too, the position detection signal will contain significant position detection error.
If significant position detection error is contained in the position detection signal, error will also be introduced in the activation start timing of the activation controlling signal, and a loss of efficiency cannot be avoided. Furthermore, error in the activation start timing of the activation controlling signal increases as the position detection error increases, leading to such problems as motor undulations and loss of synchronization due to insufficient torque, and eventually to a loss of ability to provide sensor-less drive control.
It should be noted that because the zero cross of the induced voltage can be detected during the non-activation period when sensor-less drive control using a non-activation period for rotor position detection is applied, the foregoing method enables stable sensor-less drive control. A problem with this method, however, is that this non-activation period causes vibration when the activation changes, and this results in acoustic noise.
The present invention is directed to solving the foregoing problems and an object of this invention is to provide a motor drive apparatus and motor drive method whereby vibration and acoustic noise can be reduced and a motor can be stably driven with high efficiency by sensor-less drive that does not require a non-activation period for rotor position detection.